1. Field of the Invention
The present invention relates to a belt tensioner applying a tension to a belt or a chain which is provided in a tensioned state between a drive wheel and a driven wheel.
2. Description of the Related Art
In order to apply tension to the belt or the chain (hereinafter, referred to as the belt), when the tension of the belt is suddenly increased and a plunger is to be compressed, the belt tensioner generates a damping force against the compression. When the tension of the belt is suddenly reduced and the plunger is to be expanded, the belt tensioner tends to expedite the expansion.
The belt tensioner is normally used in a so-called erect state in which a side of a cylinder is mounted to a side of a stationary member such as an engine block or the like. The cylinder is positioned at a lower side in a vertical direction and the plunger pivoting an idle pulley is positioned at an upper side. However, in accordance with a layout of a vehicle, there is sometimes a requirement that the belt tensioner is used in a so-called inverted state in which the cylinder is positioned at the upper side and the plunger is positioned at the lower side.
However, in the conventional belt tensioner (specification of Japanese Patent No. 2537547 (patent publication 1)), when it is intended to use the belt tensioner by positioning the cylinder at the upper side, a gas chamber within a casing is positioned at a side of a suction check valve for a high pressure oil chamber within the cylinder. Accordingly, the gas makes an intrusion into the high pressure oil chamber when the plunger is expanded. Thus the belt tensioner cannot be used.
Further, there is a hybrid vehicle structured such that two drive sources comprising an engine and an electric motor are wound by one belt and the drive sources are appropriately switched. In the hybrid vehicle mentioned above, there is a structure in which a belt tensioner adjusting tension of a belt is arranged at a position corresponding to a downstream side of an engine and an upstream side of an electric motor. However, when starting an engine by the electric motor in a state in which the engine is stopped, an automatic tensioner carries out a compressing operation and slip is generated in the belt. Accordingly, in order to hold the belt so as to prevent the belt from slipping, it is necessary to lock a compression stroke of the belt tensioner.
However, in the conventional belt tensioner (the patent publication 1), the damping force in a pressure side is obtained by supplying the working fluid in the high pressure oil chamber to an annular gap between an inner periphery of the cylinder and the plunger. It is impossible to close the annular gap. Therefore, it is hard to provide the lock mechanism of the compression stroke.
The applicant of the present invention has proposed a belt tensioner which can be used in the inverted state, in Japanese Patent Application No. 2002-259393. This belt tensioner is structured such that an inner tube is provided within an outer tube. A piston rod is inserted to a rod guide and an oil seal is provided in an opening portion of the outer tube. A piston rod side oil chamber and a piston side oil chamber, which is a high pressure oil chamber, are sectioned in an inner portion of the inner tube by a piston provided in a tip portion of the piston rod. An oil reservoir is provided between the outer tube and the inner tube. A discharge check valve inhibiting the oil from flowing from the oil reservoir to the high pressure oil chamber is provided in an orifice flow passage of a partition wall member provided in the inner tube, and urging means for urging the discharge check valve in a valve closing direction is provided. The urging means is placed so that the outer tube is positioned at the upper side and the piston rod is positioned at the lower side.
However, according to the belt tensioner proposed by the applicant of the present invention, since it is necessary that the rod guide, along which the piston rod slides, and the oil seal are provided in the opening portion of the outer tube, the number of the constituting parts is increased and cost becomes high. Further, since the piston is provided in the tip portion of the piston rod and a diameter of the inner tube is increased, the outer diameter of the belt tensioner increases.